1. Field of the Invention
The present invention relates to an electrochemical device and an exhaust gas purification apparatus, and more particularly, to the construction of an electrolyte layer thereof.
2. Description of the Related Art
Electrochemical devices promote chemical reactions by utilizing the ion conductivity of an electrolyte, and these devices are used in various applications. Examples of such applications include purification apparatuses (namely, electrochemical reactors), which decompose harmful components present in exhaust gas, and fuel cells, hydrogen generation apparatuses, etc.
Electrochemical devices have electrolytes between an anode and a cathode, and the properties of these electrochemical devices are influenced by the conduction properties of these electrolytes. Examples of electrolytes include those having proton conductivity that mainly operate at comparatively low temperatures (for example, about 300 to 600° C.), and those having oxygen ion conductivity that mainly operate at comparatively high temperatures (for example, about 600 to 800° C.).
FIGS. 11A and 11B show the operating principle of an exhaust gas purification apparatus using a conventional electrochemical device. FIG. 11A shows an apparatus using an electrolyte having proton conductivity, while FIG. 11B shows an apparatus using an electrolyte having oxygen ion conductivity. In both types, carbon is purified by being converted to CO2 at the anode, while nitrogen oxides (NOx) are purified by being converted to N2 at the cathode. H2O and the like are required to mediate the reaction in the case of FIG. 11A.
In this manner, in an electrochemical device of the prior art, the optimum material is selected for the electrolytes, corresponding to the temperature and other aspects of the environment in which the electrochemical device is to be used.
Japanese Patent Application Laid-open No. 2003-265931 discloses an example of this type of electrochemical device in the form of an electrochemical reactor for purifying exhaust gas. This electrochemical reactor uses an electrolyte having oxygen ion conductivity for the electrolyte.
However, electrochemical devices of the prior art had a problem since the environment in which they can be operated, was limited depending on the type of electrolyte used.
For example, a device using a proton conductive electrolyte no longer operates efficiently as temperatures rise due to a decrease in electrical conductivity. In addition, a device using an oxygen ion conductive electrolyte differs from proton conductive electrolytes in that it no longer operates efficiently as temperatures lower. Consequently, it is difficult to select a suitable electrolyte in cases requiring a wide operating temperature range.
This problem becomes particularly conspicuous in engine exhaust gas treatment apparatuses. For example, since the temperature of exhaust gas from a vehicular diesel engine extends over a wide range of 100 to 800° C., it is difficult to suitably select an electrolyte that operates over such an entire temperature range.